(1) Field of the Invention
This invention generally relates to phased array transmitters and more particularly to phased array transmitters adapted for forming steerable lobes over a wide range of operating parameters.
(2) Description of the Prior Art
Early methods for producing a steerable acoustic beam included the formation of an array of acoustic projectors powered by a single source to form a broadside beam. Steering was accomplished by rotating the entire array. These arrays tended to operate effectively only over a very narrow frequency band and were characterized by complex mechanical rotating mechanisms.
The following patents disclose arrays for producing steerable beams without requiring rotation of the array:
______________________________________ 3,324,452 (1967) Brightman et al. 4,045,800 (1977) Tang et al. 4,460,987 (1984) Stokes et al. 4,920,521 (1990) Yoshie et al. ______________________________________
The Brightman et al. patent discloses a digital phase control system that uses fixed analog or digital delays in series between a common signal source and each individual transmitter in an array for providing a steering function. The array produces a directional beam because wave energy transmitted by each of the transducers algebraically adds and reinforces the other waves in a certain direction. This direction depends solely upon the positions of the transducer in the array and the relative phase differences existing between signals applied to the adjacent transducers. In all other directions, the wave energies transmitted from each of the respective transducers of the array combine algebraically to cancel each other.
The Tang et al. patent discloses a phase steered subarray antenna adapted for providing electronic scanning while maintaining fairly low sidelobes with respect to a main lobe. Phase steering of subarrays is performed in discrete steps by means of one- or two-bit phase shifters interspersed within the feed network. The phase state of the subarray phase shifters is selected to improve the antenna gain and suppress the other lobes. Overlapping the radiating elements of the subarrays is employed to further suppress grating lobes throughout the limited scan range further.
The Stokes et al. patent discloses a variable focus sonar with a curved array. This steering of the resulting beam from the curved array is accomplished by adjusting the frequency of the transmitted signal.
The Yoshie patent discloses an array using delay time data that is received from a memory location. This time data is interpolated to provide additional information that is summed with the delay time stored in memory to obtain a final signal for transmission to each transducer in the array. This increases the effective number of time delays useful in processing the information while limiting the number of stored data points.
Each of the foregoing references discloses a phased array transmitter or beamformer that is adapted for use at a single frequency or frequency band. The following patents disclose beamformers for producing signals over a wide frequency band:
U.S. Pat. No. 4,332,018 (1982) Sternberg et al. PA1 U.S. Pat. No. 4,591,864 (1986) Sternberg et al.
In the Sternberg et al.-'018 patent an acoustic array employs a mosaic pattern acoustic lens arrangement of fully directional lens antennas as the primary array antenna elements. The lenses at the center of the array pass signals at all frequencies, the lenses near but not at the center pass all signals except those at the highest frequencies and the lenses at the outer periphery pass only those signals with the lowest frequencies. A wide band source can then supply a broad band frequency signal to a plurality of filters, time delays, amplifiers, switches and acoustic retinas for applying predetermined signals to the acoustic lenses. This array provides a constant effective aperture to wavelength ratio independent of frequency for the antenna as a whole and produces a substantially constant beamwidth, frequency independent beam. The time delays, being dependent only on the lens spacing and the scan angle and not on the wavelength, enable scanning independently of frequency.
The Sternberg et al.-'864 patent discloses a frequency independent, constant beamwidth lens antenna that produces a twisted planar or hyperbolic paraboloidal phase or wave front that in turn produces a frequency independent, constant beamwidth beam in the far field of the antenna. The frequency independent, constant beamwidth beam may be steered or scanned in azimuth without moving the lens. Specifically the antenna system includes a cylindrical lens having a longitudinal reference axis parallel to a series of generators of a cylindrical surface of a cylindrical lens and curved cylindrical focal surface. This focal surface has a longitudinal reference axis parallel to the longitudinal reference axis of the lens. A line source conforming with and located on the curved cylindrical focal surface is disposed at a transverse angle with respect to the longitudinal lens reference axis.
The foregoing patents require specially designed lenses. For example the Sternberg et al.-'018 patent requires a series of subarrays that each have special characteristics. Likewise the Sternberg et al.-'864 patent requires specially designed antenna elements in an array. Other patents that disclose the use of delay lines for shifting the signal to different elements of a transmitting array, particularly for use in ultrasonic imaging, include:
______________________________________ 4,173,007 (1979) McKeighen et al. 4,604,697 (1986) Luthra et al. 4,794,929 (1989) Maerfeld ______________________________________
The McKeighen et al. patent discloses a dynamically variable electronic delay line for controlling the phase of signals associated with an array. The time delays assure time synchronism of all transmitted pulses at a remote target.
The Luthra et al. patent also discloses an array of acoustic transducers used for ultrasonic imaging in which the signals from different transducers are delayed. The delays are selected so the presence of a target at a particular point will produce reflected envelopes that additively combine. Signals at other points will not additively combine, but will tend to cancel.
In the Maerfeld patent each part of an antenna is assigned a weighting value that can vary with frequency. The weighting according to frequency is obtained electronically or mechanically. This provides a method of steering any beam from the antenna array. The directivity, however, appears to be dependent upon the frequency of the beam and consequently not frequency independent.
Each of the foregoing references therefore discloses apparatus for controlling the transmission of acoustic energy. Each of the Brightman et al., Tang et al., Stokes et al. and Yoshie et al. patents discloses apparatus that uses fixed or variable time delays to control beam steering. However, this apparatus appears to operate only over a narrow frequency band. The Sternberg et al. patents disclose apparatus for beam steering over wide frequency bands, but this apparatus requires specially constructed antenna or transducer elements. The McKeighen et al. and Luthra et al. patents disclose phase delay circuits for controlling timing or position of a focal point, respectively. Apparatus in the Maerfeld patent provides steering as a function of frequency. None of these references, however, discloses apparatus for transmitting and steering an acoustic beam over a wide frequency band with conventional transducer elements. Consequently, each is limited in a particular application to a single narrow frequency band or to a steerable beam in which the resultant field or beam pattern varies with steering.